Particulate emissions from an engine can be reduced with a particulate filter trap and a regeneration system to periodically clean the filter trap of particulates by incineration. Generally, durable and acceptable filters for particulate traps have been developed by the art which have included wire mesh (see U.S. Pat. No. 3,499,269) and, more advantageously, rigid ceramics preferably in a honeycomb monolithic cellular wall structure (see U.S. Pat. Nos. 4,276,071; 4,329,162; and 4,340,403).
Systems of regeneration, developed by the state of the art using filters, may now be categorized as: (a) electrical heating, (b) hydrocarbon fuel fed burners, (c) catalyst impregnated traps, and (d) engine throttling. Each of the regeneration systems rely upon a heated gas to ignite the collection of particulates within the trap. The heated gas can be (1) the exhaust gas raised in temperature by high engine throttling or hydrocarbon fuel augmentation, or (2) an independent air flow raised to a high temperature by electrical heaters or fuel fed burners.
One of the problems common to all of the above regeneration systems, and associated with the use of a heated gas to ignite particulates, is the temperature gradient created in the filter trap as the result of burning of the particulate collection. As burning proceeds along the length of the filter trap, the exothermic reaction creates a continuously increasing temperature at the central core of the filter while the outer radial region of the filter trap maintains a lower temperature due to a temperature loss from radiation and conduction through the filter trap housing. It is possible to reach core temperatures that result in some destruction of the cellular or mesh filter structure.
Regeneration by electrical heating appears to be most advantageous because it offers promise of reliability and simplicity. It can be carried out under any vehicle operating condition with a minimum increase in fuel economy and without affecting emission content at the outlet of the automotive exhaust system (see use by prior art in U.S. Pat. No. 4,270,936; 4,276,066; and 4,319,896). However, these electrical regeneration systems of the prior art rely upon an unmodified flow of exhaust gas to act as the heat transfer medium between the heater elements and the particulates. The flow creates too large a mass to be quickly heated by the elements without use of oversized heating elements and without detrimental loss of energy through radiation. The heated exhaust gas intersects the filter trap in such a manner to often create detrimental temperature gradients. As a result, electrical power consumption is high and inefficient.
What is needed is a regeneration system for an automotive filter trap that has an electrical heating assembly which (a) avoids destructive temperature gradients in the filter trap during regeneration, and (b) achieves quicker regeneration through electrical means with less energy losses.